TAMPA, Fla. -- The Boston Bruins had a convincing lead. They had a substantial opportunity to completely take control of the Eastern Conference finals against the Tampa Bay Lightning. The Bruins should have won Game 4 of this series.

They did not.

Boston suffered a serious case of brain freeze when it surrendered a three-goal lead in the final two periods as Tampa scored five unanswered tallies en route to a 5-3 victory Saturday afternoon at St. Pete Times Forum.

If the Bruins hadn't flushed their lead and were able to win this game, they would return to Boston for Game 5 with a commanding 3-1 series lead.

Instead, it's dead even at two games apiece.

The Bruins let Game 4 slip away from them after holding a 3-0 lead. Now the series is tied at 2-2. Scott Audette/NHLI/Getty Images

Tampa Bay coach Guy Boucher called his team's victory a "great comeback" but expects this series to only get tougher as it shifts back to Boston for Game 5 on Monday night.

On the losing side, Bruins coach Claude Julien was not happy with the way his team completely imploded, giving the Lightning a major emotional boost in this series.

The Bruins received a pair of goals by Patrice Bergeron, including a short-handed tally, while teammate Michael Ryder also scored in the first period. It was a 3-0 cushion and the Lightning completely crushed it in the final 40 minutes of the game.

"We just lost our focus," said Julien. "The message was pretty clear. We had to continue playing the same way, but somehow we started getting stretched out again. They started getting speed. They started gaining momentum. After they scored a few goals, we almost looked like we were paralyzed out there. We weren't reacting. We weren't moving, and it snowballed from there."

What's concerning is the fact that it happened at this stage of the season. The Bruins are two wins away from advancing to the Stanley Cup finals. But so are the Lightning. This shouldn't be the case, but it is, and the Bruins need to respond from this emotional wreckage.

"Tonight, we stopped playing," Bergeron said. "We let them back in the game. It's frustrating. We didn't play the 40 minutes that we wanted. That's why we lost. But it's a long series and we can't feel sorry for ourselves because no one will. We've got to make sure to go back to what was giving us success.

"It was a perfect first period," added Bergeron. "Then we stopped battling. We stopped being hard on the forecheck, and that gave us success in the first period. In the second, we sat back."

The Bruins got complacent. They were satisfied with a three-goal lead, and Tampa took complete control of the game with traffic in front of goaltender Tim Thomas and capitalized on Boston's stale play.

"When you're up, you almost sit back a bit," said Bruins forward Brad Marchand. "You think that the game is over, and that's what we did. We took it for granted. We didn't use it to our advantage and we didn't keep pushing forward, and that's what we should have done."

The Bruins were outworked in the final two periods; Tampa completely took over. This win by the Lightning could set the tone for the rest of the series.

"It was huge," said the Lightning's Simon Gagne of the victory. "We had no choice but to win that one. We didn't want to go to Boston down 3-1. We did it against the Penguins [in the quarterfinals], but at this stage of the playoffs, we're facing a better team, a more balanced team. We knew that game was a must-win."

As much as the Bruins don't want to talk about last season and the debacle that was the Eastern Conference semifinals against the Philadelphia Flyers, when Boston forfeited a 3-0 series lead, and a 3-0 lead in Game 7, it has to be mentioned because of what occurred here Saturday. But at least this time, the Bruins have a chance to respond.

But the Bruins aren't thinking about last season or Game 4.

"I think the focus should be on winning the next game," said Thomas.

Suddenly, Game 5 is huge for the Bruins. Tampa is fired up with its comeback win and the Bruins need to respond in Boston on Monday night because if they're down 3-2 when they come back here for Game 6, staving off elimination could be a difficult task.

"The good thing about this is we've got to put it behind us and go back home and worry about that fifth game," Bergeron said. "That being said, we've got to be a lot better."

If the Bruins were told before coming here for Games 3 and 4 that they would split before heading back to Boston, the team would certainly take it. But losing a three-goal lead and this game could be extremely detrimental.

"We've seen it all playoffs. Momentum changes so many times during a game, and also during a series, so it's about us going back home and playing our game," Bergeron said. "We need to realize that first period is Bruins hockey."

The second and third periods were not, and that complacency gave Tampa a major boost in this series.